This invention relates to a minute optical lens element which is operable as an object lens system of a disk device, such as a video disk device, a laser disk device, a disk device for a computer memory, and an optical disk device which may be a magnetic optical disk device.
In general, an object lens system of the type described should have high performance and large aperture when it cooperates with the disk device because information must be accurately read from such a disk device in which information is memorized with a high density. In this connection, aberrations must be accurately corrected or adjusted in the object lens system to the extent such that a point image is determined by a diffraction limit of light. Specifically, when definition is made about the aberrations defined within a half field angle between .+-.1 and .+-.2 not only with respect to an axial ray but also with respect to an offaxial ray, the aberrations must be corrected so that they fall within a range smaller than a quarter wavelength of the diffraction limit of light.
In order to realize such a high performance lens system, conventional proposals have been mainly directed to a lens system which comprises a plurality of spherical glass lens elements and which may be called a combination lens system. However, each glass lens element and a body tube must be precisely machined or processed in such a lens system when manufacturing each lens element and the body tube. In addition, the spherical glass lens elements must be also precisely aligned with one another on assembling the combination lens system so as to prevent the spherical glass lens elements from being off center from one another. Accordingly, a number of processing stages should be accurately performed to assemble the spherical glass lens elements. This makes it difficult to reduce the cost of manufacturing the combination lens system.
Moreover, it is preferable that, when the combination lens system is used as an object lens system, the object lens system is light in weight and small in size. This is because the object lens system should respond to a high speed access operation which is carried out during tracking or focusing. However, the body tube inevitably becomes long in structure and the combination lens system becomes heavy in weight. Thus, it is difficult to make the combination lens system light in weight and small in size.
Recent attempts have been directed to an object lens element which is formed as a single minute optical lens element in a pickup optical system of an optical disk device. Such a single optical lens element should have high performance and the large aperture, like the combination lens system when it is used as the object lens system. Specifically, the single optical lens element should have resolving power which is equal to or less than 1 micron meter.
As such a single optical lens element, a compact and light-weight lens system has been disclosed by T. Nakamura in U.S. Pat. No. 4,571,034, assigned to Olympus Optical Co., Ltd. The compact and light-weight lens system is formed by a single aspherical lens element which has a focal length f, a refractive index n, and entrance and exit aspherical surfaces determined by predetermined formulae, respectively. Each of the predetermined formulae is dependent on a conical coefficient k of each surface, and a radius of curvature r of each surface. More specifically, the conical coefficients k.sub.1 and k.sub.2 of the entrance and the exit surfaces are defined by: EQU -2.2&lt;k.sub.1&lt; 1.1 and (1) EQU 0.8&lt;k.sub.2&lt; 1.2, respectively. (2)
In addition, a factor of (n-1)f/r.sub.1 falls within a range between 0.6 and 1.4, both exclusive, where r.sub.1 is the radius of curvature of the entrance surface.
Herein, it is mentioned here that a center or an optical axis of the entrance surface is often offset from that of the exit surface when manufacturing such a single aspherical lens element. In this event, it has been found that a coma acutely takes place in the single aspherical lens element disclosed in the above-referenced patent. This is because the coefficient p.sub.2 expressed by the formula (2) must fall within a very narrow range. Accordingly, a good yield of production can not be accomplished when the single aspherical lens element is manufactured.
Moreover, such a single aspherical lens element is practically formed or manufactured from a plastic material by the use of injection molding. It is to be noted that an index of refraction and a dispersion ratio of the plastic material are locally distributed within a very narrow range on a diagram of an index-dispersion ratio and that the plastic material is also inferior to glass material in homogeneity. Therefore, it is difficult to obtain a desired index of refraction and a desired dispersion ratio as long as plastic material is used to form the single aspherical lens element. This means that the degree of freedom is restricted in optically designing such a lens element when plastic material is used.
Besides, plastic material is inferior to glass material in heat resistance, a moisture resistance, temperature characteristic, and weather resistance. Under the circumstances, such a single aspherical lens element is not always suitable for the object lens element of an optical disk device because the above-enumerated characteristics must be satisfied in the object lens element.